There are several human autosomal recessive syndromes known to be associated with cancer or leukemia. While the homozygous affected individuals are rare, the clinically unaffected heterozygotes are relatively common in the general population, occurring at a frequency of between one in 100 and one in 1,000 for the syndromes in question. If a heterozygous carrier for one of these genes has a risk of dying from a malignant neoplasm two or more times the normal risk, the gene may be regarded as an important neoplasia-predisposing gene in man. As association between malignant neoplasms and each of these genes can be detected by a retrospective study of the causes of deaths in families with homozygous probands, since heterozygotes are frequent in such families. Finding such associations may provide useful leads for further investigations of oncogenesis in man.